Method and apparatus for monitoring channel frequency response

ABSTRACT

A physical layer protocol is added to retrieve the in-band downstream ADSL channel frequency response H(f), the noise N(f), measured at initialization and the signal to noise ratio SNR(f) measured at show time on a per bin basis. Additionally retrieval of similar in-band information may be provided in the upstream direction. The definition of the message protocol for retrieving during show time the following ATU-R information on a per bin basis: In-band channel frequency response per bin H R (f) measured during the initialization referred back to the receiver tip and ring copper pair by the ARU-R; In-band noise estimation per bin N R (f) during the initialization referred back to the receiver tip and ring copper pair by the ATU-R; and the signal-to-noise ratio per bin SNR R (f) during show time referred back to the receiver tip and ring copper pair by the ATU-R. The values of SNR R (f) should be updated as they change. An addition of the programming interface in the ADSL ATU-C chipset level makes similar information available for the upstream direction, that is N c (f), N c (f) and SNR c (f). Initialization H(f) can be used for analyzing the physical copper loop condition between tip and ring. Initialization N(f) can be used for analyzing the crosstalk. Showtime SNR(f) can be used for analyzing time dependent changes in crosstalk levels and line attenuation (such as due to moisture). The combination of H(f), N(f) and SNR(f) can be used for trouble shooting why the data rate cannot reach the maximum data rate of a given loop, scheduling maintenance and plant update.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and apparatus fordetermining, storing and retrieving channel frequency characteristicsfor discrete multi-tone (DMT) and is particularly concerned withasymmetric digital subscriber lines (ADSL) and very high bit-ratedigital subscriber lines (VDSL) using DMT modulation.

BACKGROUND OF THE INVENTION

[0002] Digital Subscriber Line (DSL) is a well known access technologythat uses existing 2-wire copper telephone wiring (also known asunshielded twisted pair UTP) to deliver high-speed data services tohomes and businesses. DSL technology has become popular with bothsubscribers and Internet service providers because the service uses thecustomer's existing phone line and typically does not require anadditional phone line. In addition, the signaling used by some DSLtechnology is above that used by plain old telephone service (POTS).Hence, this allows an “always-on” Internet access while still providinguse of the phone line. Current ADSL technology offers users a choice ofspeeds up to about 8 Mbps. This is much faster than a standard 56 Kbpsdial-up modem.

[0003] There are many types of DSL, generically these have come to beknown by the designation xDSL. For any given line, for a particular typeof xDSL, the maximum xDSL speed is limited by the frequency response ofthe channel which is a function of the distance between the subscriberand the central office (CO) and the filtering placed on the line, and bythe noise conditions on the line which are primarily a function ofcrosstalk from other signals in the cable and ingress from AM radiotransmitters. On long loops speed frequency response and noiseconditions of the channel bound the upper rate. On shorter loops thexDSL technology and its allocated bandwidth may be bit rate determiningfactor.

[0004] Asymmetric Digital Subscriber Line (ADSL) is one of the xDSLtechnologies that provide more bandwidth in one direction than theother, typically downstream from the central office to the subscriber.Very high bit-rate Digital Subscriber Line (VDSL) is a particular typeDSL that may be configured as an asymmetric or a symmetric serive. Itdelivers from 13 to 52 megabits per second downstream bandwidth and 1.5to 13 megabits per second upstream. VDSL may be implemented using singlecarrier or DMT based modulation. ADSL uses a form of modulation known asdiscrete multi-tone modulation (DMT). DMT is method of modulation thatdivides the available frequency range into sub-channels or tones, thenumber of sub-channels depends on the particular implementation.

[0005] ITU-T Recommendation G.992.1 also known as G.dmt, is a form ofADSL technology, using DMT modulation, that offers up to 8 MBPSdownstream bandwidth, 1.544 MBPS upstream bandwidth. ITU-TRecommendation G.992.2 is also known as G.lite, is a form of ADSLtechnology, using DMT modulation, that offers up to 1.5 MBPS downstreambandwidth, 384 KBPS upstream. The rates mentioned above are not caps onthe achievable data rate in these technologies but reflect ratesdiscussed in the ITU standards documents.

[0006] The deployment of xDSL services in the copper loops helpedservice providers in providing new services and creating new revenues.However, the introduction of these services also created new challengesto the service providers in the maintenance and diagnostics area.Traditional methods of determining that a twisted pair is suitable forPOTs service are not adequate for these higher frequency services. Therate determining factors of channel frequency response and noise canchange over time, and require monitoring to ensure adequate service.These challenges include how to provide effective and low costmaintenance operation for the new services.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide an improvedmethod and apparatus for determining, storing and retrieving channelcharacteristics for discrete multi-tone.

[0008] Accordingly, the present invention provides method and apparatusfor retrieving channel characteristics measured at a CO end of thechannel by a CPE end of the channel. If the channel characteristics isdetermined at the CPE end and retrieved at the CO end, the first end ofthe channel means the CPE end and the second end of the channel meansthe CO end. However, if the channel characteristics is determined at theCO end and retrieved at the CPE end, the first end of the channel meansthe CO end and the second end of the channel means the CPE end.

[0009] To assist the ADSL service providers to meet the challengesdiscussed above, an embodiment of the present invention adds a physicallayer protocol to retrieve the in-band downstream ADSL channel frequencyresponse H(f), the noise N(f), measured at initialization and the signalto noise ratio SNR(f) measured at show time on a per bin basis.

[0010] An embodiment of the present invention also provides forretrieval of similar in-band information in the upstream direction.

[0011] Conveniently, an embodiment of the present invention adds thefollowing parameters to G.992.1 bis and G.992.2 bis.

[0012] (a) The definition of the message protocol for retrieving duringshow time the following ATU-R information on a per bin basis:

[0013] i. In-band channel frequency response per bin H_(R)(f) measuredduring the initialization referred back to the receiver tip and ringcopper pair by the ARU-R.

[0014] ii. In-band noise estimation per bin N_(R)(f) during theinitialization referred back to the remote tip and ring copper pair bythe ATU-R.

[0015] iii. The show time signal-to-noise ratio per bin SNR_(R)(f). Thevalues of SNR_(R)(f) should be updated as they change.

[0016] (b) The addition of the programming interface in the ADSL ATU-Cchipset level to make similar information available for the upstreamdirection, that is N_(c)(f), N_(c)(f) and SNR_(C)(f). The conditions ofthem must be the same as the above.

[0017] (c) H(f), N(f) and SNR(f) are proposed as mandatory parameters.

[0018] The purposes of making the above information available are:

[0019] a) Initialization H(f) can be used for analyzing the physicalcopper loop condition between tip and ring.

[0020] b) Initialization N(f) can be used for analyzing the crosstalk.

[0021] c) Showtime SNR(f) can be used for analyzing time dependentchanges in crosstalk levels and line attenuation (such as due tomoisture).

[0022] d) The combination of H(f), N(f) and SNR(f) can be used fortrouble shooting why the data rate cannot reach the maximum data rate ofa given loop, scheduling maintenance and plant upgrade.

[0023] The invention is described mainly for ADSL. However, it may besimilarly applicable to VDSL.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention will be further understood from thefollowing detailed description with reference to the FIGURE in which:

[0025] FIGURE illustrates in a functional block diagram an asymmetricaldigital subscriber line in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Referring to the FIGURE there is illustrated in a functionalblock diagram an asymmetrical digital subscriber line (ADSL) inaccordance with an embodiment of the present invention. The ADSL 10connects a customer premise equipment (CPE) 11 to a central office (CO)12. The CPE 11 includes an ADSL modem 14 (ATU-R), POTS splitter 16, acomputer 18 and a telephone 20. The CO 12 includes an ADSL modem 22(ATU-C), a POTS splitter 24, and a telephone switch 26. The ADSL 10effectively includes the ADSL modems 14 and 22 and the unshieldedtwisted pair (UTP) 28 there between. The ADSL arrangement of the FIGUREis intended as merely representative and person skilled in the art ofDSL would appreciate that POTS splitter 16 can be centralized as shownor distributed throughout the premise and attached to each telephoningdevice telephone, facsimile, or answering machine. Actual connectionswould depend on the version of ADSL used for example G-dmt or G-lite.

[0027] In operation, the customer connection via the ADSL 10 includes aninitialization stage and a show time stage. The embodiments of thepresent invention enhance ADSL service maintenance and diagnostics bymaking diagnostic information available from both ends of the loopduring active operation of the service. The combination of completeinformation on the channel (H(f) and N(f)) during initialization plusshow time SNR(f) is provided by the embodiment of the FIGURE. Thiscombination of data allows greater analysis of the line conditions thanknown methods and reduces interruptions of both the ADSL and POTSservice that known methods require.

[0028] Embodiments of the present invention allow obtaining of usefulinformation about the channel in which data is being transmitted.

[0029] An example of how the present invention could be implemented isnow provided for the purposes of illustration. The embedded operationschannel (EOC) defined in G.992. 1 and G.922.2 are for communicationbetween ATU-C and ATU-R. The EOC supports in-service and out-of-servicemaintenance and the retrieval of ATU-R status information andperformance monitoring parameters (G.992.2, Section 8.1). As theproposed parameters are for maintenance diagnostics and performancemonitoring, it is logical to add these parameters in the ATU-R dataregisters, so they can be retrieved through the EOC.

[0030] In G.992.1, registers 9 ₁₆ and F₁₆ are reserved for future useand in G.992.2, register A₁₆ is currently being used. In order to applythe proposed registers to both G.992.1 and G.992.2 consistently, usingregister B₁₆ and E₁₆ is proposed. TABLE 9-3 G.992.1-ATU-R data registersReg. # Use Length Description 0₁₆ Read(R) 8 bytes ATU-R vendor ID (see9.3.3/ G.994.1) 1₁₆ R Vendor ATU-R version number discretionary minusone 2₁₆ R 32 bytes ATU-R serial # 3₁₆ R Vendor Self test resultsdiscretionary 4₁₆ Read/Write Vendor Vendor discretionary (R/W)discretionary 5₁₆ R/W Vendor Vendor discretionary discretionary 6₁₆ R 1byte Line attenuation 7₁₆ R 1 byte SNR margin 8₁₆ R 30 bytes ATU-RConfiguration_((see 8.4) and Note 1) 9₁₆ Reserved_((Note 2)) A₁₆ Beingused in G.992.2 B₁₆ R 1024 bytes The normalized channel frequencyresponse H_(R)(f) at ATU-R C₁₆ R 2 bytes The scale factor HS_(R) ofH_(R)(f) D₁₆ R 256 bytes The noise N_(R)(f) at ATU-R E₁₆ R 256 bytesSNR_(R)(f) at ATU-R F₁₆ Reserved Reserved See Note 2

[0031] The in-band frequency channel response, H_(R)(f), is representedby a normalized complex number a(i) +jb(i) in the linear format, where 1is the subcarrier index i=0, . . . , 255. Both a(i) and b(i) are codedas a two-byte 2's complement signed fixed point value respectively. Thedata format of a(i) and b(i) are the same. Bit 15 is a sign bit. Thedecimal point is on the right of bit 15. The accuracy is 1/32768. In theregister, the msb is stored as the left byte and the Isb is stored asthe right byte. The value of a(i)+jb(i) must be referred to tip and ringof the copper loop.

[0032] The first two bytes in H_(R)(f) register store the real part (a)of a(i)+jb(i) for bin 0 and the second two bytes store the imaginarypart (b) of a(i)+jb(i) for bin 0 and so on. The last four bytes storesthe a(i)+jb(i) values for bin 255 for G.922.1 or bin 127 for G.922.2. Avalue of (−1−j) in a(i)+jb(i) is a special value. It indicates that thisbin is either out of the downstream PSD mask or this bin is not used forthe data transmission (eg it may be because of the attenuation is toolarge, so that the SNR is too small or it is the DC subcarrier or theNyquist subcarrier). The data range is between −32767/32767/32768inclusive. The register reserves space for all 256 bins, as it is easyfor application user to remember.

[0033] It is suggested that the reported channel frequency responsevalues be partitioned between the scale factor and per carrier complexcomponents such that max(|a(i)|, |b(i)|) over all i is equal to32767/32768 in order to maximize precision.

[0034] The H_(R)(f) register is only updated during the ADSLinitialization stage. This information can be retrieved during showtime, however, it may not be updated during show time.

[0035] The scale factor HS_(R) is coded as two bytes 2's complementunsigned fixed point value. The decimal point is on the right of bit 15.One bit is used for integer and 15 bits are used for the fraction. Theaccuracy is 1/32768. In the register, the msb is stored as the left byteand the 1 sb is stored as the right byte. The data range is between 0and +(1+32767/32768) inclusive. There is no special value for it.

[0036] This data range supports a dynamic range of approximately +6dB to−90dB. The portion of the scale factor range above zero is necessary toaccommodate the possibility that on short loops, given manufacturingvariations in signal path gains and filter responses, it is possiblethat the channel may appear to have gain rather than loss.

[0037] The HS_(R) scale factor register is only updated during the ADSLinitialization stage. This information can be retrieved during showtime, however, it may not be updated during show time.

[0038] The conversion of the normalized value of a or b to the absolutevalue can be obtained by multiplying a or b by HS_(R).

[0039] The in-band noise, N_(R)(f), is coded as a one-byte unsignedinteger. The unit is in dBm/Hz. The accuracy is 1 dBm/Hz. Since thenoise in dBm/Hz is a negative value, a value of 95 means−95dBm/Hz. Thenoise values must be referred to the tip-ring of the copper loop.

[0040] The first byte in N_(R)(f) register stores the value for bin 0and the second byte stores the value for bin 1 and so on. The last bytestores the value for bin 255 for G.922.1 or bin 127 for G.992.2. A valueof 255 is a special value. It indicates that this bin is either out ofthe downstream PSD mask or this bin is not used for the datatransmission (e.g. It is the DC subcarrier or the Nyquist subcarrier).The data range is between 0 and 254 inclusive, where the highest value255 is reserved as the special value.

[0041] The N_(R)(f) register is only updated during the ADSLinitialization stage. This information can be retrieved during showtime, however, it may not be updated during show time.

[0042] The in band SNR_(R)(f) is a one-byte unsigned integer. The unitis in dB. The accuracy is 0.5dB. For example, a value of 00000001represents 0.5dB and a value of 01000001 is 64.5dB.

[0043] The first byte in the SNR_(R)(f) register stores the SNR valuefor bin 0 and the second byte stores the SNR_(R)(f) value for bin 1 andso on. The last byte stores the SNR_(R)(f) value for bin 255 for G.922.1or bin 127 for G.922.2. A value of 127.5 is a special value. Itindicates that this bin is either out of the downstream PSD mask or itis not used for the data transmission (e.g. it is the DC subcarrier orthe Nyquist subcarrier). The data range is from 0 to 127 dB inclusive,where the highest value 127.5 is reserved as the special value.

[0044] The SNR_(R)(f) register is updated during the ADSL show time. Itis updated as changes occur.

[0045] The upstream channel frequency response H_(C)(f), the scalefactor HS_(C), the noise N_(C)(f) and the signal to noise ratioSNR_(C)(f) are available in the ARU-C. Currently, no programminginterface is available at the chipset level to allow for diagnosticspurposes of the ADSL service. The data definitions and their formatsshould be the same as the ones proposed for the downstream direction inthis contribution.

[0046] With the existing information available from ITU G.992.1 andG.992.2 as well as from embodiments of the present invention, the ADSLmaintenance and diagnostic capabilities can be enhanced.

[0047] The retrieved information, in at least one embodiment, can beused for remotely analyzing the physical copper loop condition betweentip and ring. That is for:

[0048] analyzing the crosstalk at both ATU-C and ATU-R, i.e. analyzingtime dependent changes in crosstalk levels and line attenuation (such asdue to moisture);

[0049] trouble shooting why the data rate cannot reach the maximum datarate of a given loop.

[0050] The above capabilities would otherwise require a truck roll plusthe use external test equipment.

[0051] The disclosed embodiments propose a new way to collectinformation available in the ADSL modems, such that this information canbe used for enhancing the ADSL services maintenance and diagnostics. Forthe information required from ATU-R, the embodiments of the presentinvention save the information (per bin frequency channel response, perbin signal to noise ratio and per bin noise) in the ATU-R register. Theinformation saved in the register is the frequency channel response andnoise information as measured during the modem initialization. Thisinformation is reflected back to the tip and ring of the copper pair.The signal to noise ratio information is the show time information. Afurther embodiment also provides the provision of the programminginterface to allow for retrieval of the same information on the ATU-Cside.

What is claimed is:
 1. A method of retrieving channel characteristicsfor a discrete multi-tone communication channel comprising the steps of:at initialization, determining and storing channel frequency responseand noise measurements at a CO end of the channel; at show time,determining and storing a signal-to-noise measurement at said first end;at a CPE end of the channel, retrieving at least one of the storedmeasurements; and receiving data at the CPE end at a rate in dependenceupon the one or more of the retrieved measurement.
 2. A method asclaimed in claim 1 wherein the channel is symmetrical.
 3. A method asclaimed in claim 1 wherein the channel is asymmetrical.
 4. A method asclaimed in claim 1 wherein the channel is overlapping.
 5. A method asclaimed in claim 1 wherein the channel is non-overlapping.
 6. A methodas claimed in claim 1 wherein the channel is ADSL.
 7. A method asclaimed in claim 1 wherein the channel noise is N(f).
 8. A method asclaimed in claim 1 wherein the channel frequency response is H(f).
 9. Amethod as claimed in claim 1 wherein the signal-to-noise measurement isSNR(f).
 10. An apparatus for retrieving channel characteristics for adiscrete multi-tone communication channel comprising: a circuit for atinitialization, determining and storing channel frequency response andnoise measurements at a CO end of the channel; a circuit for at showtime, determining and storing a signal-to-noise measurement; a circuitfor at a CPE end of the channel, retrieving at least one of the storedmeasurements; and a circuit for transmitting to the CPE end at a rate independence upon the retrieved measurement.
 11. An apparatus as claimedin claim 10 wherein the channel is symmetrical.
 12. An apparatus asclaimed in claim 10 wherein the channel is asymmetrical.
 13. Anapparatus as claimed in claim 10 wherein the channel is overlapping. 14.An apparatus as claimed in claim 10 wherein the channel isnon-overlapping.
 15. An apparatus as claimed in claim 10 wherein thechannel is ADSL.
 16. An apparatus as claimed in claim 10 wherein thechannel noise is N(f).
 17. An apparatus as claimed in claim 10 whereinthe channel frequency response is H(f).
 18. An apparatus as claimed inclaim 10 wherein the signal-to-noise measurement is SNR(f).
 19. A methodof retrieving channel characteristics for a discrete multi-tonecommunication channel comprising the steps of: at a CO end of thechannel, retrieving from a CPE end of the channel, at least one of thechannel frequency response, noise measurement or signal-to-noisemeasurement that were previously determined and stored at said CPE end;and, at the CO end, receiving data from said CPE end, at a rate independence upon one or more of the retrieved measurements.
 20. A methodas claimed in claim 19 wherein the retrieved channel frequency responsewas determined and stored at said CPE end during the initialization ofsaid CPE end.
 21. A method as claimed in claim 19 wherein the retrievednoise measurement was determined and stored at said CPE end during theinitialization of said CPE end.
 22. A method as claimed in claim 19wherein the retrieved signal-to-noise measurement was determined andstored at said CPE end during show time at said CPE end.
 23. A method ofretrieving channel characteristics for a discrete multi-tonecommunication channel comprising the steps of: at a CPE end of thechannel, transmitting to a CO end of the channel, at least one of thechannel frequency response, noise measurement or signal-to-noisemeasurement that were previously determined and stored at said CPE end;and, at the CPE end, sending data from to said CO end at a rate independence upon one or more of the transmitted measurements.
 24. Amethod as claimed in claim 23 wherein the channel frequency response isdetermined and stored at said CPE end during the initialization of saidCPE end.
 25. A method as claimed in claim 19 wherein the noisemeasurement is determined and stored at said CPE end during theinitialization of said CPE end.
 26. A method as claimed in claim 19wherein the signal-to-noise measurement is determined and stored at saidCPE end during show time at said CPE end.
 27. A method comprising thesteps of: retrieving the in-band downstream or upstream ADSL channelfrequency response H(f), the noise N(f), as measured at initializationand the signal to noise ratio SNR(f) measured at show time on a per binbasis.
 28. An apparatus comprising: a register for storing measurementsof in-band downstream or upstream ADSL channel frequency response H(f),the noise N(f), measured at initialization and the signal to noise ratioSNR(f) measured at show time, respectively on a per bin basis; and acircuit for retrieving measurements of in-band downstream or upstreamADSL channel frequency response H(f), the noise N(f) and the signal tonoise ratio SNR(f) during show time.
 29. A method comprising the stepsof: retrieving the in-band downstream or upstream discrete multi-tonebased VDSL channel frequency response H(f), the noise N(f), as measuredat initialization and the signal to noise ratio SNR(f) measured at showtime on a per bin basis.
 30. An apparatus comprising: a register forstoring measurements of in-band downstream or upstream discretemulti-tone based VDSL channel frequency response H(f), the noise N(f),measured at initialization and the signal to noise ratio SNR(f) measuredat show time, respectively on a per bin basis; and a circuit forretrieving measurements of in-band downstream or upstream discretemulti-tone based VDSL channel frequency response H(f), the noise N(f)and the signal to noise ratio SNR(f) during show time.